1. Technical Field
The subject invention relates generally to braking systems and, more specifically, to fluid-operated brake actuators such as air or pneumatic brake actuators.
2. Description of the Prior Art
Fluid-operated braking systems, such as air brake systems, have long been used to control movement of motor vehicles in a safe and effective manner. In particular, air brakes are commonly used on commercial vehicles such as trucks, trailers and buses, which typically have large gross vehicle weights. The considerable inertial mass of these heavy-duty vehicles in combination with the high speeds at which they travel requires a braking system which responds rapidly with substantial braking power. One system component which is instrumental in the operation of air brake systems is the brake actuator. The brake actuator provides the force necessary when braking the vehicle. The assignee of the preset invention produces a number of high-quality commercial air brake actuators.
One such prior art brake actuator is a spring brake actuator shown at 10 in FIG. 1. The prior art spring brake actuators 10 typically have a network of air chambers 12, 14 defined by flexible diaphragms 20, 22 and the respective housing components. Typically, the top air chamber 12 is an emergency chamber and the bottom or service chamber 14 is a working chamber. The service chamber 14 includes a push rod 24 having a head or piston 26 which is biased against the diaphragm 22 by return spring 28. When the brake is actuated by the operator, the inlet port 30 is pressurized by the pneumatic pressure of the vehicle, reversing the cup-shaped flexible diaphragm 22 which reacts against the piston 26, driving the push rod 24 through an opening 32 in the end wall of the service chamber housing 34 as shown by arrow 36, actuating the braking system of the vehicle. The emergency chamber 12 includes a piston or push rod 38 having a head 40 which is biased against the diaphragm 20 by return spring 42. In a spring brake actuator of the type shown, the emergency chamber further includes a power spring 44 which is spring biased against the head or cover 46 and the spring side piston 48. The pneumatic pressure system of the vehicle acting through port 50 counterbalances the force of the power spring 44, such that the power spring is normally collapsed as shown in FIG. 1. However, when the pneumatic pressure system of the vehicle fails or the parking brake is actuated, the pressure in the emergency chamber 12 returns to atmospheric pressure and the power spring 46 expands against the piston 48, collapsing the cup-shaped diaphragm 20 and driving the piston 38 through an opening 52 in the web 54 of the flange case 56, driving the push rod 24 through the opening 32 in the housing 34, actuating the braking of the vehicle. Thus, the emergency chamber 12 serves as an emergency brake in the event that the pneumatic system of the vehicle fails or a parking brake.
The push rod 24 is connected to the braking system of the vehicle by a bifurcated yoke 58 and locking pin 60 as described below. The push rod 24 is externally threaded into an internal threaded opening in the yoke 58 (not shown) and retained in place by lock nut 62. The overall length of the push rod 24 and yoke 58 assembly is important to proper actuation of the braking system. The opening 32 in the end wall of the service chamber 34 must be wide enough to accommodate lateral or skewed motion of the push rod, as shown. This opening 32 in the disclosed embodiment is protected by a stone shield 64 and the cup-shaped retainer plate 66. The stone shield 64 is permitted to move laterally in the retainer plate 66, as shown. The stone shield 64 is conventionally formed of a relatively hard polypropylene to resist stones and debris during normal operation of the vehicle as described below.
As would be understood by those skilled in this art, brake actuators are normally attached to mounting plates on the underside of the vehicle chassis. Thus, the brake actuators are subject to extreme conditions including road debris, ice, snow and extreme temperature variations. Typically, dirt and ice collects on the yoke 58, locking pin 60, lock nut 62 and the threads on the push rod 24. Further, dirt and ice can penetrate the opening 32 in the end wall of the service chamber housing 34 accumulating on these various components. This accumulation of ice and debris may interfere with the movement of the push rod or prevent lateral movement of the push rod as described. Further, the externally threaded push rod wears the internal opening through the stone shield 64, reducing the life of these components. Another disadvantage of the yoke assembly shown is the number of components which must be accurately assembled to control the overall length of the assembly. Tests of this assembly have shown that the stroke length may be reduced as much as 15 mm., which can adversely affect braking performance. Another deficiency is the metal to metal contact between the yoke 58 and the locking pin 60 which directly interconnects the yoke with the metal control arm (not shown). After repeated use, the metal to metal contact may become increasingly worn and subject to fatigue.
There is therefore a need for an improved push rod and yoke assembly which is of simple and cost-effective design, easy to install and which avoids accumulation of road debris and ice. Another object would be to reduce the overall weight of the push rod and yoke assembly. Finally, the push rod and yoke assembly should have an attachment device which is less stressful on the corresponding parts.